The present invention relates generally to G-Rake receivers, and more particularly to the estimation of combining weights for a G-Rake receiver.
Conventional Rake receivers for Code Division Multiple Access (CDMA) systems exploit multipath reception for improved signal-to-noise ratios. In operation, each of two or more Rake fingers obtains despread values from a received CDMA signal by correlating the received signal against a known spreading sequence. By aligning the processing delay of each Rake finger with a different path delay of the multipath signal, the Rake receiver effectively obtains a different copy of the desired signal for each Rake finger. Maximum ratio combining of the despread values from each Rake finger yields, at least in theory, a combined signal having an improved signal-to-noise ratio as compared to the signal from any one Rake finger.
The “standard” Rake receiver works well in white noise environments, where the signal impairments are uncorrelated across the Rake fingers. Standard Rake receiver performance becomes sub-optimum in colored noise environments, where at least some components of the overall received signal impairments may be strongly correlated across the Rake fingers. In other words, the standard Rake receiver does not perform well in terms of suppressing colored interference, where the received signal impairments across Rake fingers may exhibit significant correlations.
The “Generalized Rake” (G-Rake) receiver was developed to better suppress interference by taking into account impairment correlations across Rake fingers. The basic operation of a G-Rake receiver is described in Bottomley, et al., Generalized Rake Reception for Cancelling Interference from Multiple Base Stations,” IEEE Vehicular Technology Conference (2000) and U.S. Pat. No. 6,363,104 B1 to Bottomley et al. The best interference suppression is achieved if up-to-date covariance information (the “instantaneous color” of the impairment) is used when determining the combining weights. U.S. Pat. No. 6,714,585 B1 to Wang et al. describes a method for computing G-Rake combining weights based on received signal impairment correlations.
Rather than directly calculating received signal impairment correlations, it is known to represent received signal impairments according to a parametric model that is dynamically “fitted” to ongoing observations of impairment, which may be short-term, somewhat “noisy” snapshots of received signal impairment. In parametric G-Rake receivers, an overall received signal covariance matrix is constructed based on available channel information and is expressed as the combination of various constituent components of the impairment. The relative weights (fitting parameters) of these components are determined dynamically, such as by fitting the model terms to ongoing impairment correlation measurements. U.S. Published Application No. 2005/0201447 A1 to Cairns et al. describes an exemplary parametric G-Rake receiver.
One problem with the parametric G-Rake receiver is the estimation of the scaling parameters. In parametric G-Rake receivers, the performance of the receiver in terms of interference suppression depends on the accuracy of the fitting parameters. The introduction of higher-order modulation and multiple-input, multiple-output in Release 7 of the WCDMA standard will increase the signal-to-interference ratio (SINR) at the receiver and will make parameter estimation less reliable. Therefore, there is considerable interest in finding and developing new techniques for obtaining reliable estimates of scaling parameters in a parametric G-Rake receiver.